CCITT Study Group XV (Working Party XV/1) is charged with transmission systems. Under WP XV/1
a Specialists Group was established dealing with drafting recommendatioms for the secomd generation
sub-primary rate ( n x 384 kbit/s, ii = 1, . . . , 5) or (p x 64 lcbit/s, p = 1, . . . , 30) video codecs.
The Specialists Group on Coding for Visual Telephony reaches their objectives by ezchanging the
results with the different partners involved (Europe, Japan, USA, Korea and Canada). During the study
period 1984-1989 the Specialists Group agreed upon the usage of a so called reference model (hereafter
abbreviated RM) for simulation purposes. The specification for a fiezible hardware is derived from these
simulations. In this paper a description of the reference model and the evolution towards the last reference
model (RM8) is given which is the basis for the H.261. The intentions of this contribution is to show the
ft ezibility of the algorithm for different applications. The term universal approach makes therefor reference
to the usage of the algorithm for a range of possible applications. In an joint expert group of ISO/IEC,
the Moving Picture Coding Expert Group (MPEG) ISO/IEC JCT/5C2/WG8, work is carried out to select
a standard for coded represenation of moving images and sound for the provision of interactive moving
picture applications. In this expert group, members of the specialists group on coding for visual telephony
are participating trying to realize interworking between the standard in preparation by the joint expert group
MPEG and the coming CCITT recommendation H.261. For the most important used techniques in the
CCITT Reference Model, among which are quantization, scanning, loop filter, entropy coding, multiple
versus single VL C and block type discrimination theoretical information is provided and some examples for
possible improvements are included. A significant development is reported i.e a modification of the reported
n x 384 kbit/s algorithm paving the way to a universal standard codec capable of operating at p x 64 kbit/s (
p = 1,...,30).